Fix typo in CSSOM-1 (#11996)

Author: Cameron-Asdf

cssom-1/Overview.bs

@@ -2824,7 +2824,7 @@ follow these rules:
     * If either of the above syntactic translations would be less backwards-compatible,
         do not perform them.
 
-    Note: The rules described here outlines the <em>general principles</em> of serialization.
+    Note: The rules described here outline the <em>general principles</em> of serialization.
     For legacy reasons, some properties serialize in a different manner,
     which is intentionally undefined here due to lack of resources.
     Please consult that property's specification and/or your local reverse-engineer for details.